Electromagnetic hammers having ferromagnetic masses of conventional type, e.g. as described in document JP-A-56 153018, comprise a tube carrying a coil and, in the vicinity of one of its ends, an anvil. A mass of ferromagnetic material is slidably received inside the tube.
The coil is generally made by winding a copper cable onto an associated former. After the coil has been wound, the former together with its coil is fixed on the tube.
In use, the substantially vertical or inclined tube rests via its anvil on the element to be hammered. The coil is then excited by electrical power supply means, thereby generating an electromagnetic field that raises the mass. Thereafter, excitation of the coil is interrupted and the mass, under the action of its own weight, strikes the anvil which transmits the shock to the element that is to be hammered.
However, it has been found that while the mass is being raised, the coil which is not closely and rigidly supported is itself subjected to a reaction force that tends to compact it. In use, these successive deformations of the coil reduce the performance of the electromagnetic hammer and can lead to the coil being damaged.
Another problem encountered is that since the mass is very heavy and needs to strike the anvil on several occasions in order to drive home the element concerned, the electrical power supply means need to deliver a very high instantaneous level of power repetitively and for a very short duration. Unfortunately, when such means are connected to an electrical power supply mains, the large current peaks drawn suddenly and repeatedly from the mains give rise to voltage drops that are repeated at short intervals, giving rise to a "flicker" effect. The user then has to take out a subscription corresponding to the maximum level of power consumption, even though this level can be several times the mean level of power consumption. When the power supply means comprise an independent generator, then the current peaks drawn from the generator give rise to variations in the speed of the engine of the generator. This gives rise to significant wear on the engine and to an abnormal amount of exhaust fumes being produced.
Documents FR-A-2 015 204 and FR-A-2 581 100 illustrate electromagnetic hammers of the above-described type, in which the means for delivering electrical power to the coil comprise a battery of capacitors. The capacitors accumulate energy in electrostatic form and release it in a very short length of time. The capacitors suffer from the drawback of being expensive and of having a lifetime that is relatively short because of the high rate at which they are charged and discharged. The electrical circuit is also generally complicated.
In certain uses, it is necessary for the electromagnetic hammer to deliver shocks of lower energy and at a high frequency. Unfortunately, the capacitors behave like short circuits at the beginning of charging so they must be associated with a current limiting choke and a resistor so as to limit the current drawn from the mains. In addition, the capacitors and the coil form an oscillating circuit whose resonant frequency determines the maximum frequency of the blows. To obtain a higher frequency, it is necessary to reduce the number of capacitors. That kind of operation is not suitable for being performed on a worksite. In addition, the power of the blows and their frequency are varied by varying the voltage to which the capacitors are charged. Such variation can sometimes be difficult due to the complexity of the circuit.